The role of the feline oncornavirus-associated cell membrane antigen (FOCMA) in association with the malignant state will be investigated. The studies will be done using both spontaneous leukemia or lymphoma cells and fibroblasts transformed in vitro by the feline sarcoma virus. Of particular interest are leukemia cells from "virus negative" cats which express none of the virus structural proteins. The cross reactivity of FOCMA expressed on sarcoma virus transformed cells will be compared with FOCMA expressed on leukemic cells, and cats exposed to only the feline leukemia virus will be evaluated for reactivity to FOCMA induced by the sarcoma virus. Whether or not FOCMA is induced in nonfeline species (especially dogs) when feline leukemia virus is injected will be determined. The relationship between expression of FOCMA and the cell cycle as well as the relationship between FOCMA and cell surface histocompatibility will also be studied.